The present invention relates generally to Sagnac fiber-optic rotation-sensing interferometers and more particularly to a Sagnac fiber-optic rotation-sensing interferometer which produces an alternating output signal.
The Sagnac ring interferometer, which employs a long single-mode-optical-fiber loop, has shown promise as a passive gyroscope for navigational purposes. The properties of Sagnac fiber-optic gyroscopes are described in an article by Rashleigh and Burns entitled "Dual-Input Fiber-Optic Gyroscope", Optics Letters, Vol. 5, No. 11, p. 482, Nov. 1980. Typical Sagnac fiber-optic gyroscopes include means for introducing counter-propagating light beams into both ends of a fiber-optic loop and photodectors for measuring the intensities of the output light beam. This intensity detection then gives a measure of the relative phase shift between the counterpropagating beams in the fiber-optic loop in order to determine the rate of rotation about an axis perpendicular to the plane of the fiber-optic loop. An optical coupler/divider splits an incoming light beam into the counter propagating beams in the fiber-optic loop and later combines the output light beam exiting the fiber-optic loop, allowing them to interfere.
Two problems associated with the operation of typical interferometers are low sensitivity at near-zero rotation rates and the effect of noise on intrinsic dc operation.
Recently several interferometers have been developed which operate at maximum sensitivity, or quadrature, for near-zero rotation rates. However, these devices produce a dc signal which limits sensitivity due to high intrinsic, low-frequency noise levels in the photodetectors.
Since noise generally has a broad power spectrum, the power of the noise at a given frequency, for example f.sub.o, is low. If the output signal is periodically varied at f.sub.o and amplified by a phase-locked amplifier, then much of the noise is eliminated and an improvement of several orders of magnitude in the sensitivity of the interferometer is achieved. Also, since noise is superimposed upon the output signal, it is desirable to achieve a signal of high average power in order to maximize the signal to noise ratio and to minimize loss of sensitivity.
Several methods of producing an alternating output signal from an interferometer have been developed whereby the intensity of the input beam to the interferometer is periodically varied. However, since power is switched into and out of the interferometer the average power level of the output signal is low and sensitivity is not maximized.